Epitaxy is used in integrated circuit technology to create single-crystal layers, or films, on top of single-crystal substrates such that the resistivity and dopant type of the layer can be adjusted independently of the resistivity and dopant type of the substrate. These films are used in such integrated circuit technologies, such as, Bipolar, I.sup.2 L, CMOS, VMOS and Silicon on Sapphire, as well as in the fabrication of power, microwave imaging and solar-energy-conversion devices.
Silicon (Si), because of its excellent physical properties, is the primary element utilized in fabricating integrated circuits. Although other elements or compounds may also be used, the description herein will purposely refer to silicon, because of its present importance.
Silicon epitaxial films are conventionally grown using atmospheric pressure Chemical Vapor Deposition (CVD) [M. L. Hammond, Solid State Technology, 22, 61 (1979)]at temperatures of 1050-1150.degree. C. To accommodate the small cell sizes required for future VLSI designs, very thin (0.5-1.5 .mu.m), lightly doped epitaxial films on top of heavily doped substrates are required. However, there are severe technological limitations that prevent the fabrication of such films [R. Reif and R. W. Dutton, J. Electrochem. Soc., 128, 909 (1981)].
When lightly doped epitaxial films are deposited on heavily doped substrates at the high temperatures used by conventional processes, a considerable transfer of dopants from the substrate to the growing film takes place. This transfer results in a transition region extending 1.5-2.5 .mu.m into the epitaxial layer within which the doping level changes only gradually with position. The existence of this transition region clearly prevents the fabrication of extremely thin, lightly doped epitaxial films.
The transfer of dopants is caused by two mechanisms: solid-state diffusion, which is caused by the random thermal motion of atoms, and autodoping, which is caused by the redeposition of dopants that have evaporated from surfaces in the reactor, such as those of the wafers and the susceptor [G. R. Srinivasan, J. Electrochem. Soc., 127, 1334 (1980)]. By growing epitaxial films at reduced pressures (40-100 Torr) [R. B. Herring, "Semiconductor Silicon", R. R. Hanerecht and E. L. Kern, eds., Electrochem. Soc., 126 (1979)], the autodoping problem is ameliorated; however, the most important deposition parameter controlling dopant redistribution is temperature. Therefore, by lowering the process temperature, thermal diffusion and impurity evaporation are reduced and thus more abrupt transitions can be obtained [R. Reif and R. W. Dutton, J. Electrochem. Soc., 128, 909 (1981)]. Furthermore, lower pressure and lower temperature have also been shown to reduce pattern shift and distortion [E. Krullman and W. L. Engl, IEEE Trans. Electron. Devices, ED-29, 491 (1982)] and wafer warpage [H. H. Steinbeck, 150th Electromechanical Soc. Meeting, Abs. 352, October 1976], respectively, both of which create alignment problems for small-scale lithography.
A lower process temperature for epitaxial films is clearly desirable for future processes; however, the lower temperature creates two new problems that must be considered. In the conventional process, silicon's native oxide is removed by hydrogen reduction at high temperatures to expose the silicon lattice. Next, gaseous species are transported to the wafer surface where they undergo thermally-driven chemical reactions. The resulting adatoms, atoms that have been adsorbed on the surface, must then migrate to energetically favorable surface sites to be incorporated into the crystalline lattice. Lowering the process temperature complicates the removal of the native oxide and it reduces the surface mobility of the adatoms, both of which make epitaxial growth more difficult, if not impossible.
Low temperature epitaxial films have been grown using silicon molecular beams [Y. Ota, J. Appl. Phys., 51, 1102 (1980); T. Takagi, I. Yamada and A. Sasaki, J. Vac. Sci. Tech., 12, 167 (1982); P. C. Zalm and L. J. Beckers, Appl. Phys. Lett., 54, 1466 (1983)]. The higher the degree of ionization of these beams, the lower the temperature at which epitaxial films can be grown. Using a completely ionized beam, silicon epitaxial films have been deposited at 125.degree. C. [P. C. Zalm and L. J. Beckers, Appl. Phys. Lett., 41, 167 (1982)]. However, molecular beam technology, because of the required relatively high vacuum, will probably prove to be impractical in a high volume manufacturing environment.
W. G. Townsend et al. [W. G. Townsend and M. E. Uddin, Solid State Elec., 16, 39 (1973)] reported epitaxial growth of silicon at temperatures in the range of 800.degree.-1150.degree. C. Their wafer was immersed in a 300 watt hydrogen plasma prior to the deposition. Silane was added to begin the deposition, which was done at a total pressure of 0.6 Torr with a 1:10 silane:hydrogen mixture. Townsend et al. attained epitaxy at temperatures as low as 800.degree. C. Plasma enhancement during deposition was required to sustain epitaxial growth at 800.degree. C.
Suzuki et al. [S. Suzuki and T. Itoh, J. Appl. Phys., 54, 1466 (1983) and S. Suzuki, H. Okuda, T. Itoh, Proc. 11th Congress (1979 International) on Solid State Devices, 647 (1974)], use a low temperature plasma enhanced CVD to obtain epitaxial films at 750.degree. C. The wafer is not immersed in the plasma (i.e., it is below the plasma region) and the cleaning procedure uses a GeH.sub.4 plasma. The in-situ cleaning procedure is not explained well, but it is believed the radicals from the GeH.sub.4 plasma reach the wafer and react with the native oxide to form the volatile species GeO.sub.2 which can be pumped away. However, some germanium is also deposited on the wafer resulting in a 5-7% germanium concentration in the first 1000 .ANG. of the silicon epitaxial film. In their most recent work, Suzuki et al. [S. Suzuki and T. Itoh "Epitaxial Growth of Si-Ge Layers on Si Substrates by Plasma Dissociation of SiH.sub.4 and GeH.sub.4 Mixture" J. Appl. Phys. 54(11), (November 1983)] leave the GeH.sub.4 flow on during the entire deposition to form a 5-7% Ge/Si alloy throughout the epitaxial film.
A need therefore still exists for a low pressure CVD process which can deposit uniform epitaxial films at temperatures below 800.degree. C. with or without plasma enhancement and which does not introduce unwanted concentrations of elements in the film.